A fisheries ecologist at the University of Alaska Fairbanks, Krista Oke has heard it time and again from fishermen up and down Alaska rivers: Salmon just aren’t as big as they used to be. This is especially true on the Yukon River, she says, where Chinook salmon is a cultural touchstone and staple food for Indigenous people in both the US and Canada.
In recent decades, rising river temperatures in the Yukon watershed have ushered in a steady decline in Chinook abundance. Now, researchers have confirmed that these fish are significantly smaller as well. “You have fewer fish coming back, and each of them is smaller,” says Oke. “There could be really important consequences of these changes.”
In a study published in August in Nature Communications, Oke and a team of researchers analyzed 60 years’ worth of data collected by the Alaska Department of Fish and Game. The dataset included measurements of age and length for 12.5 million individual salmon, covering four species — Chinook, coho, chum, and sockeye — across all regions of the state. The study affirmed anecdotal observations. Salmon are shrinking.
The extent of size reductions varied among species and regions. Chinook showed the greatest decline — on average, they declined by 8 percent — followed by coho salmon at 3.3 percent, chum at 2.4 percent, and sockeye at 2.1 percent.
Shrinking fish has been an increasingly common phenomenon in an ocean altered by a changing climate and dominated by human activity. And there could be many reasons why it’s happening. Scientists studying cod and zebrafish have shown that fishing that targets larger fish can meddle with natural selection, leading to a reduction in maximum body size in fished populations.
Other fish may be shrinking as a direct result of climate change. Marine biologists Daniel Pauly and William Cheung have posited a principle called the “gill oxygen-limitation theory,” which suggests that warmer oceans could be straining fishes’ metabolism. High-energy fish, like bluefin tuna, could be growing more slowly as a result.
In Alaska, Oke and her colleagues considered size-selective fishing and metabolic processes to explain salmon size decline. But their data showed that Alaskan salmon are not necessarily growing more slowly than they did historically. Rather, these fish are spending fewer years at sea than they used to. They are returning to freshwater younger, and therefore smaller.
“Those older age classes just aren’t present in populations anymore,” says Eric Palkovacs, an evolutionary ecologist at the University of California, Santa Cruz and a co-author of the study.
Scientists call this process age truncation, and it’s happening with many fish species around the world, like Pacific herring, European perch, and various reef fish. There are a few reasons why it happens, but for Alaska salmon, according to Palkovacs, they all point to the fact that the ocean is becoming an inhospitable place for older, larger fish. “The ocean is a riskier place to be,” he says.
Size-selective fishing is one hypothesis, but Palkovacs says that their dataset didn’t include enough fishery data to come to a firm conclusion. However, intensified predation on some species of salmon by nonhuman predators could be operating in a similar way.
A paper published last year by a team at the University of Washington says that a growing population of killer whales on the Pacific coast selectively feeds on large Chinook salmon. The paper suggests that the conservation success of marine mammals like orcas may be resulting in “the selective removal of large fish and an evolutionary shift toward faster growth and earlier maturation” among salmon. The lower abundance of Chinook populations further intensifies the risk of mortality for an individual older fish.
Competition among salmon is another factor. According to Palkovacs, there are historically high numbers of pink and chum salmon in the North Pacific. Researchers from the National Marine Fisheries Service recently published that pink salmon production has also increased with a warming Bering Sea, while hatcheries increasingly pump pink and chum into Alaskan waters.
“There’s a limited amount of food in the ocean,” says Palkovacs. “There’s strong evidence that these record high numbers of chum and pink salmon causes food stress and competition.”
For salmon, competition and predation equate to risk of mortality. That risk is intensified when food sources are impacted by warm periods like marine heatwaves, which are becoming more common with climate change. The result of all these factors: Adult salmon err on the side of caution, and abort their seafaring years early.
The consequences of smaller salmon can be significant, according to Oke. “Salmon are incredibly important in Alaska, for both people and ecosystems,” she says. For the commercial salmon fishery in Alaska, which brought in $744 million in 2018, smaller fish fetch lower profits. More importantly, smaller salmon could pose a food security risk for subsistence fishers. “[Smaller fish] means fewer meals going into their freezers, in an area where store bought replacements are extremely expensive,” says Oke.
Oke also explains that smaller salmon could have an impact on riparian ecosystems. When salmon spawn and die, their carcasses provide nutrients to land mammals and plants. Plus, bigger fish lay more eggs. When fish shrink, their per-capita egg production declines as well, with adverse effects on salmon survivability.
Just as there’s no single “smoking gun,” as Oke puts it, for why salmon and other fish are shrinking, solutions to reverse the trend can be elusive. But Oke, Palkovacs, and their co-authors explain that this research could have implications for how salmon are managed.
Alaska salmon management adheres to an escapement policy that determines a fixed number of adult salmon that reach their spawning grounds each year. As abundance fluctuates, harvest numbers fluctuate with it, while the number of fish that “escape” to spawn remains constant. Now, that fixed escapement number doesn’t paint a complete picture of spawning salmon. Characteristics of each fish, like size and age, should matter too.
Another takeaway from the study, according to Palkovacs, concerns how we learn to recognize the extent of our impact on a planet undergoing human-caused climate change — and in an ocean that was once thought to be boundless.
“Alaska’s the most pristine North American salmon producing region, [yet] we’re seeing evidence of population specific size declines,” he says. “Nowhere is really totally pristine.”
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